1. Field of the Invention
The present invention relates to a noise control apparatus for an internal combustion engine and, more particularly, to a noise control apparatus for obtaining a desirable characteristic of sound including noise elimination by producing a noise control wave that interferes with noise (engine intake and/or exhaust noise) generated by the operation of the engine.
2. Description of the Related Art
To reduce noise, for example, intake noise generated by the intake system of an internal combustion engine, a noise eliminator, such as a resonator, is conventionally disposed in an intake duct. However, to meet the today's growing demand for quietness, a plurality of resonators of increased size are required, taking up an increased installation space in an engine compartment or a vehicle body structure. Moreover, the noise reducing effect of such a conventional system is not sufficient despite the increased number and size of resonators.
Recently, employment of an open control system which uses pre-stored map data regarding phase and sound pressure has been proposed for a noise control apparatus, wherein based on the map data, an actuator (speaker) is caused to produce a noise-control wave for interference with intake noise
However, the intake noise reduction rate achieved by such conventional noise control apparatuses undesirably varies with changes in the operational conditions of an internal combustion engine and maximum noise reduction cannot be attained. More specifically, the conventional noise control apparatuses are unable to produce, over a wide rage of the engine operational conditions, an optimal noise control wave that is equal in sound pressure level but opposite in phase, i.e., shifted by 180 degrees, with respect to instant intake noise, which depends on various engine operational conditions. The conventional apparatuses determine a noise control wave to be produced, based on the engine speed and load. However, the sound pressure level of intake noise varies with changes in the intake air temperature even when the engine speed remains unchanged, as shown in FIG. 13, which shows sound level-engine speed curves at two different intake air temperatures of 16 and 32 degrees Celsius. The phase of intake noise also varies with changes in the intake air temperature even when the engine speed remains constant. Thus, the conventional noise control apparatuses fail to produce the optimal noise control waves and accordingly fail to achieve largest-possible noise reduction when the intake air temperature changes.